Watt linkages and quadrilaterals

We define a Watt quadrilateral to be a quadrilateral with a pair of opposite sides of equal length. See Figure 1. 2. Linkages The Watt linkage (Figure 2) has equal-length cranks AD and BC , A and B fixed, and coupler bar CD . It was devised by James Watt about 1784 to constrain the steam-engine piston rod, connected at E , the midpoint of CD , to approximate straight-line motion over a limited range.